Most sulfur in the refinery motor gasoline pool generally comes from FCC gasoline. The FCC gasoline (or “naphtha”) can be hydrotreated to remove sulfur. However, FCC gasoline tends to be olefinic, and conventional hydrotreating can often result in too large of an octane loss, due to near complete olefin saturation. Selective hydrotreating processes have been developed, e.g., SCANfining, to maintain higher relative hydrodesulfurization with reduced (optimally minimal) olefin saturation, by a combination of specific catalyst and operation in a narrow range of optimized operating conditions. Since the operating window in SCANfining can tend to be narrow, any contaminants to the process can tend to be very significant in this technology.
One potential source of contaminants into the hydrotreatment process can be from makeup hydrogen, typically from a steam-reforming hydrogen plant or from a catalytic reformer. It has been found that some of these streams can contain carbon monoxide in surprisingly high levels, which can act to suppress activity/selectivity in the FCC gasoline hydrotreating process, requiring higher required reactor temperatures to overcome this suppression. Carbon monoxide can also tend to buildup in the recycle gas system, such that the effective concentration in the reactor is higher than the concentration in the makeup hydrogen itself. Higher operating temperatures both narrow the operating window (resulting in lower cycle length) and saturate more olefins (resulting in higher octane loss). In order to prevent deactivation of the catalysts and/or reduction in the process hydrodesulfurization levels, in conventional SCANfining processes, the carbon monoxide contents of the naphtha feedstream, and particularly the hydrogen gas streams, to the SCANfining reactor(s) have been maintained at target carbon monoxide levels to less than 5 vppm.
Carbon dioxide can additionally be present in makeup hydrogen streams. CO2 generally has less effect itself, as most selective FCC gasoline HDS units have amine recycle gas scrubbers that remove CO2 in the recycle gas. However, it is known that some CO2 will be converted to CO over many hydrotreatment catalysts.
Thus, it would be desirable to identify catalysts that are tolerant of carbon monoxide (and/or carbon dioxide) and/or that convert less carbon dioxide to carbon monoxide during the hydrotreatment process. Included below are methods of utilizing such catalysts in methods where carbon oxides are prevalent, in order to improve the effectiveness and/or efficiency of the methods, e.g., for making motor gasoline and perhaps other fuels/petroleum products as well.